Lunar Dust Buster

April
19, 2006: Ever get a fragile item packed in a box
filled with Styrofoam peanuts? Plunge your hands into the
foam peanuts to search for the item, and when you pull it
out foam peanuts are clinging to your arms. Try to brush them
off, and they won't fall off—instead, they seem to hop away,
only to cling to your legs or elsewhere. The smaller the peanuts,
the more tenacious they seem. In fact, if you break a foam
peanut into bits, the tiny lightweight bits are almost impossible
to brush off.

This
behavior is classic static cling.

It's
also the behavior of lunar—and possibly also Martian—dust.

The
dozen Apollo astronauts who landed on the Moon between 1969
and 1972 found moondust to be an unexpected challenge. Not
only was it so abrasive that it wore partially through the
outer gloves of their space suits, but also it stuck to everything.
The more they tried to brush it away, the more it worked its
way into the space suits' fabric.

Right:
A tiny, jagged speck of moondust. Micro-photograph courtesy
of David McKay, NASA/JSC.

Part
of the dust's tenacious clinging was due to the sharp, irregular
shapes of individual dust grains, formed by millions of years
of meteorite impacts that repeatedly melted rocks into glass
and then broke the glassy rocks into powdered glass. The particles'
jagged edges were almost like claws that hooked into things
like microscopic burrs.

But
another reason was the dust's electrostatic charge. On the
Moon, harsh, unshielded ultraviolet rays from the Sun have
enough energy to kick electrons out of the upper layers of
the regolith (soil), giving the surface of each dust particle
a net positive charge. The smaller the particles, the less
their mass and the greater their charged surface area, so
the more they clung—just like Styrofoam peanuts broken into
small bits.

A
team led by Carlos I. Calle (lead scientist at NASA's Electrostatics
and Surface Physics Laboratory at Kennedy Space Center), however,
has used a bit of intellectual judo to figure out a way to
take advantage of the dust grains' electrostatic charge to
repel them. In fact, they've come up with a new application
of an old idea.

"In
the 1970s, electrical engineering professor Senichi Masuda
of the University of Tokyo—well-known as a pioneer in electrostatics—came
up with the 'electric curtain'," Calle recalls.

Masuda,
not even thinking about the Moon or Mars, was working on air
pollution filters. Because smog particles are often charged,
Masuda came up with a prototype of what he called the electric
curtain.

Essentially,
the electric curtain was a series of parallel electrodes—thin
copper wires—spaced roughly a centimeter (half an inch) apart
in a circuit board. To them, Masuda applied alternating current,
such as that from an ordinary wall socket. The word "alternating"
refers to the fact that the electrons in the wires are quickly—60
times a second in the United States, 50 times a second in
Europe—made to move forward and backward along the wire, instead
of just in one direction as in the direct current from a battery.

But
instead of providing the same alternating current to all the
parallel electrodes at once, Masuda did something ingenious.
He slightly delayed the onset of the current to each successive
electrode. That slight delay made the electromagnetic field
of each electrode to be out of phase with its neighbors, creating
an electromagnetic wave that rapidly traveled horizontally
across the surface on which the electrodes lay. Moreover,
any charged particles lying on the surface got lifted and
moved by that traveling electromagnetic wave, as if they were
surfers being pushed along by an ocean wave.

Fast
forward to the present: After seeing how Martian dust that
collected on the solar panels of Mars Pathfinder deprived
it of electric power and impaired performance, Calle and his
collaborators began wondering if the electric curtain could
be adapted to keep solar panels on the Moon and Mars dust-free.
After all, he reasoned, "human and robot astronauts can't
always be cleaning windows."

To
let sunlight through, however, the device would need transparent
electrodes. So instead of using copper wires, Calle and his
colleagues made electrodes out of indium titanium oxide (ITO),
transparent semiconducting oxides "that are now a mature
technology, used in the touch screens of PDAs (personal digital
assistants)," Calle explains. They also moved the electrodes
closer together—just a few millimeters apart. The result was
a transparent film that "is flexible, like a sheet of
vinyl," that they call an electrodynamic dust shield.

When
the transparent dust shield was covered with simulated lunar
or Martian dust (which is mostly ground-up volcanic ash and
cinders from terrestrial volcanoes) and put in a vacuum chamber
that was then pumped down to the rarefied atmospheric pressure
of Mars or the Moon, it worked like a champ. Most of the dust
was thrown off to the side in seconds: movie.

None
of the simulated lunar and Martian soils, however, contains
either the pure iron or shards of glass found in real moondust.
This difference might affect the dust shield's effectiveness.
"I'm hoping to try the experiment again with a vial of
actual lunar dust," he says.

While
Calle is optimistic about the dust shield's effectiveness
for large flat or gently curved surfaces such as solar panels
and the visors of astronauts' helmets, "folds in the
fabric of a space suit are more challenging," he says.
"ITOs are flexible, but after a point will break."
He's also working to fine-tune the voltage, frequency, phase,
and other electrical properties of the traveling electromagnetic
wave to repel the greatest amount of dust. And he's curious
to see how it would behave in lower gravity.

There's
much work to be done, says Calle, but one day his device might
draw the curtain on the troubles with moondust.